The present invention relates to an apparatus and method for scanning or pointing the beam of a phased-array antenna via electronic control. More particularly, it relates to an apparatus and method for distributing electromagnetic energy to output ports of a planar antenna array and controlling the time delay between a common input port and any one of multiple output ports by distributing controllable time-delay elements in the pattern of a fractal tree within the antenna feed network.
Microwave and millimeter-wave systems, such as air-SATCOM communication links, have been continuously increasing in complexity and density of components due to consumer demands. The increasing number and variety of components, controllers, and connections have correspondingly increased power consumption and may contribute to noise and other interference problems in these systems. The beamformer, an integral component of any such system, has not remained unaffected.
Beamformers (or electronically scanned arrays) may be fabricated in one- or two dimensions. One example of a conventional beamformer for a one-dimensional phased-array antenna is shown in FIG. 6. The conventional beamformer 100 contains an input port 102 to which an electromagnetic signal is fed, transmission lines 104, phase control devices 106 or phase shifters, and output ports 108. The transmission lines 104 are arranged at a power splitter 103 such that the electromagnetic signal from the input port 102 is divided into a plurality of signals with equal or unequal power. The phase shifters 106 adjust the phase of these signals in accordance with control signals 112 provided from an external controller (not shown). Each control signal 112 is provided to an individual phase shifter 106 and may either tune the phase difference of the phase shifter 106 or simply turn on the phase shifter 106 thereby applying a set amount of phase difference. The output ports 108 are connected to radiating elements 110 (e.g. antennas) that transmit the various phase-shifted signals to an external system (not shown). The combination of the phase-shifted signals emitted from the antennas 110 forms an amplitude profile/aperture of the overall beamformer 100.
The phase shifter 106 simulates a time delay for a signal that passes through the phase shifter 106 by altering the phase of the signal. The different phases forming the aperture effectively point the signal through the radiating element 110 at a specific pointing angle or direction toward receiving elements in the external system. To an observer, the phase delays make the signal appear as if it is effectively scanned in time across the output ports 108 at that particular frequency. Conventional phase shifters 106 are typically individual devices that are soldered or fixed into a circuit board, such as PIN diodes (with hybrid circuitry) or other types of ferrite-based devices. As shown, such a conventional beamformer 100 employs one phase shifter 106 at each radiating element 110.
However, conventional beamformers suffer from a number of problems. One disadvantage is that phase shifters are lumped elements and are thus external to the substrate containing the feed network or the antenna array. The phase shifters are thus relatively bulky and expensive. Phase shifters are also generally RF-active devices that require a comparatively large amount of power and may interfere with the transmitted signal. Another disadvantage is that, because the phase shifter alters the phase of an input signal thereby only simulating a time delay, a fixed, progressive time delay between elements is obtained only over a relatively narrow band of frequencies. As a consequence, if the frequency of the beam wanders, the pointing angle wanders correspondingly. For example, using current phase shifters, for high-gain beams, having a gain of around 10 dB, stringent requirements exist: the bandwidth of signals able to be transmitted or received within acceptable margins is only about 5-10%. For low-gain beams, having a gain of around 15 dB, the requirements are somewhat less severe to produce an acceptable beam: the bandwidth may be about 20-30%.
Thus, the beamformer which employs phase shifters only forms a beam at essentially one frequency or a narrow band of frequencies; if the frequency transmitted changes substantially, the antenna element spacing must be either physically moved or the phases set by the phase controllers changed to form a beam at the new frequency (in a controllable-type beamformer array). This process may be time consuming and awkward. Alternatively the process may be physically impossible. Further, this is increasingly important for systems communicating at frequencies that are relatively far apart, some existing and proposed earth-orbiting satellite communication systems communicate simultaneously at approximately 20 and 30 GHz.
Furthermore, as shown, conventional beamformers employ one phase-shifter localized at each radiating element. Thus, a controllable beamformer requires one control signal per antenna element, with associated computer, signal processing, control lines, and control line multiplexing hardware. The resulting beamformer and antenna control unit are typically bulky and extremely expensive, and, as mentioned above, can only form a beam at one frequency.
Accordingly, it would be advantageous to produce a compact, planar, low-cost electronically-controllable high-gain array that can form and steer a beam whose pointing angle is constant at multiple frequencies, or over a broad band of frequencies. Further, it would be advantageous to produce an electronically controllable beamformer in which the pointing angle is controlled using a reduced number of control signals, thereby decreasing the complexity of the control electronics.
The embodiments of the beamformer comprise an input port that is configured to receive an input electromagnetic signal, output ports that are configured to provide output electromagnetic signals, and controllable time delay elements that are disposed between the input port and the output ports. The time delay elements are distributed in a multi-branched feed network, which includes a fractal tree.
Each time delay element may be controlled by an analog voltage or current signal or may be controlled by a digital signal.
The time delay elements may be controlled by fewer control signals than the number of time delay elements.
The fractal tree may comprise a base (or initiator) pattern including a first set of the time delay elements connected symmetrically with the input port and branch (or generator) patterns symmetrically connected with the initiator pattern. Each generator pattern may include a second set of the time delay elements and be connected with a set of the output ports. Or the generator pattern in the fractal tree may be recursively connected to yet another stage of generator patterns in the fractal tree structure. Unique control signals that control the time delay elements may be equal to 1-2 signals per dimension of beam scanning, for example: beam scanning in 1 dimension may require only 1-2 signals while beam scanning in 2 dimensions may require only 3-4 signals. The fractal tree may be symmetrically arranged around the input port.
Each generator pattern of the fractal tree may be substantially identical and may have substantially identical numbers of time delay elements and time delay elements have substantially identical time delays. Similarly, the time delay elements of the initiator pattern and generator patterns may be substantially identical or different in time delay and/or placement.
The beamformer may comprise only (radio frequency) RF-passive components. The beamformer may be integrated with printed-circuit antenna elements and may comprise an integrated, monolithic system on a printed circuit board.